Delayed coking is one of several types of process used in oil refineries to convert heavy oils to useful lighter products. Essentially, it is a carbon rejection process in which the hydrogen:carbon ratio of the heavy oil feed is increased to form lower boiling products with a higher hydrogen content by eliminating the excess carbon in the form of the coke product. In delayed cokers, the heavy oil feed is preheated in the same fractionation tower (the coker combination tower) used to separate the cracking products into differently boiling fractions. This pre-heated feed, together with any recycled bottoms from the combination tower, is then fed into a continuously operating process furnace to effect a limited extent of thermal cracking, after which it enters a large, vertically-oriented cylindrical vessel or coking drum, in which the major portion of coking reactions take place. Usually, two or more drums are fed by a single furnace so that the drums can be filled and emptied in sequence while running the furnace continuously, making this a semi-batch process. In the coke drum, large oil molecules are further thermally cracked to form additional lighter products and residual coke, which fills the vessel. The lighter hydrocarbons flow out of an outlet at the top of the drum as vapor and are further processed into fuel products after passing through a coker combination tower from which a bottoms stream may be withdrawn for recycle with the fresh feed. Gradually the coke accumulates in the drum until it is almost filled with coke. When the drum is nearly filled, the hot oil from the furnace is directed to a clean coke drum, while the full one is decoked. The decoking cycle involves cooling, depressuring and draining water from the drum, purging it with steam to remove residual hydrocarbon vapor, opening up the top and bottom heads (closures) on the drum and then using high pressure water lances or mechanical cutters to remove the coke from the drum. The coke falls out the bottom of the drum into a pit, where additional water is drained off and conveyers take the coke to storage or rail cars. The drum is then closed up and is ready for another coking cycle.
The feedstocks for delayed cokers are typically the heaviest (highest boiling) fractions of crude oil that are separated in the crude fractionation unit, normally comprising an atmospheric distillation tower and vacuum tower. The nature of the coke formed is highly dependent on the characteristics of the feedstock to the coker as well as upon the operating conditions used in the coker. Although the resulting coke is generally thought of as a relatively low value by-product, it may have some value, depending on its grade, as a fuel (fuel grade coke) or for electrodes for aluminum or steel manufacture (anode grade coke). Generally, the delayed coker is considered to produce three types of coke with different morphologies that have different appearances, properties and economic values. Needle coke, sponge coke, and shot coke are the most common along with transitional forms. Needle coke is the highest quality of the three varieties which commands a premium price; upon further thermal treatment, needle coke has high electrical conductivity (and a low coefficient of thermal expansion) and is used to make the electrodes in electric arc steel production. It is low in sulfur and metals and is frequently produced from some of the higher quality coker feedstocks that include more aromatic feedstocks such as slurry and decant oils from catalytic crackers and thermal cracking tars. Typically, it is not formed by coking of resid type feeds. Sponge coke, a lower quality coke, is most often formed in refineries from lower quality refinery coker feedstocks having significant amounts of asphaltenes, heteroatoms and metals. If the sulfur and metals content is low enough, sponge coke can be used for the manufacture of anodes for the aluminum industry. If the sulfur and metals content is too high for this purpose, the coke can be used as fuel. The name “sponge coke” comes from its porous, sponge-like appearance. Conventional delayed coking processes, using the vacuum resid feedstocks, will typically produce sponge coke, which is produced as an agglomerated mass that needs an extensive removal process including drilling and water-jet cutting technology.
Shot coke is considered the lowest quality coke. The term “shot coke” comes from its spherical or ovoidal shape bail-like shape, typically in the range of about 1 to about 10 mm diameter. Shot coke, like the other types of coke, has a tendency to agglomerate, especially in admixture with sponge coke, into larger masses, sometimes larger than a foot in diameter. This can cause refinery equipment and processing problems. Shot coke is usually made from the lowest quality high resin-asphaltene feeds and makes a good high sulfur fuel source, particularly for use in cement kilns and steel manufacture. There is also another coke, which is referred to as “transition coke” and refers to a coke having a morphology between that of sponge coke and shot coke. For example, coke that has a mostly sponge-like physical appearance, but with evidence of small shot spheres beginning to form as discrete shapes. The term “transition coke” can also refer to mixtures of shot coke bonded together with sponge coke.
In the semibatch delayed coking process, the drum is filled with the heated feed until the coke bed, typically at a temperature of 425° C. or higher has filled a drum; the coke mass must then stripped of cracking products with steam and then cooled and cut from the drum using high-pressure water jets. The coke cooling and cutting steps can take several hours per 12-15 hour cycle, and frequently the cooling is nonuniform, presenting hazards during the cutting operation. If 100% free-flowing (nonbonded) shot coke were produced, no cutting/drilling would be required and a significant reduction in cycle could be achieved along with a corresponding increase in unit throughput resulting in an increase in the production of liquid hydrocarbon products which are the economic drivers of the process. The production of shot coke can therefore be regarded as economically desirable regardless of the low value of the coke by-product.
Articles in the technical literature by Siskin and Kelemen together with their colleagues have provided insights into the possibilities of controlling coke morphology. See, for example. Siskin et al, “Asphaltene Molecular Structure and Chemical Influences on the Morphology of Coke Produced in Delayed Coking”, Energy & Fuels 2006, 20, 1227-1234; Siskin et al, “Chemical Approach to Control Morphology of Coke Produced in Delayed Coking”, Energy & Fuels, 2006, 20, 2117-2124; Kelemen et al, “Delayed Coker Coke Morphology Fundamentals: Mechanistic Implications Based on XPS Analysis of the Composition of Vanadium and Nickel-Containing Additives During Coke Formation,” Energy & Fuels 2007, 21, 927-940. In addition, a series of patents and applications from ExxonMobil Research and Engineering Company presented different proposals for promoting the production of a free-flowing shot coke during the delayed coking process; publications of these include U.S. Pat. Nos. 7,374,665; 7,871,510; 03/048271; 2007/050350; 2004/104139; 2005/113711; 2005/113712; 2005/113710; 2005/113709; 2005/113709; 2005/113708; 2007/058750.
While previous work was successful in enabling the morphology of the delayed coke product to be controlled by reference to the metals (Ni, V, Na, K) content of the resid it did not define the limits bracketing the shot coke formation window in a wholly quantitative or mechanistic manner so as to provide a higher degree of certainty in the formation of a free-flowing shot coke. A fuller understanding of the interfacial surface effects required for more definitive control of the coke morphology will, however, provide the refiner with higher certainty for forming shot coke that allows more reliable operation and enhanced safety of operation for instance, by enabling throttling drum closure valves to be fitted on the coke discharge ports at the bottom of the coke drum as described in U.S. 2005/0269247. Continuous operation of the delayed coker may also become possible with adequate control of the coke morphology as described in U.S. Pat. No. 7,914,668.